Don't feed them shit like stereotypical shounen anime, that will only give them bad taste or so to speak, tunrning them to being intolerant to every other anime out there. Start out with those "short" yet baddass episode animes with a good start and good action like Trigun/Hellsing/Samurai Champloo.

Then you can move on to longer animes with morals like Deathnote/Full metal alchemist/Monster.
After that they're probably ready to follow even longer series like D.gray man, Gintama and One piece.

If they like a certain sport it'd be easy to just start off with that. Or if they 're dirty suckers for fanservice just show them any harem/ecchi show out there.